This invention relates generally to improvements in bearing systems for use in high speed rotating machinery or turbomachines, particularly such as a turbocharger for supplying charge air at elevated pressure to an internal combustion engine. More particularly, this invention relates to an improved floating bearing cartridge having a pair of angular contact bearings for rotationally supporting a high speed rotary shaft, wherein the angular contact bearings are slightly unloaded, or subjected to a slight negative pre-load to enhance smooth-running operation with minimal bearing wear while accommodating radial and alternating thrust loads, thermal loads, and transient condition loads during normal operation.
Turbochargers are well known in the art for use in supplying charge air under pressure to an internal combustion engine for the purpose of increasing engine performance. Such turbochargers generally comprise a turbine wheel and a compressor wheel or impeller mounted on a common shaft which is supported by suitable bearings for high speed rotational operation. The turbine wheel is positioned within a turbine housing shaped for flow-through passage of engine exhaust gases which rotatably drive the turbine wheel at relatively high speed. The thus-driven shaft and associated bearings are typically mounted within a so-called center housing disposed between the turbine housing, and a compressor housing having the compressor impeller therein. Accordingly, the exhaust-gas driven turbine wheel rotatably drives the compressor wheel which draws in and compresses ambient air to provide pressurized charge air to the associated internal combustion engine.
Significant design and development effort has focused upon the turbocharger shaft bearings in attempts to provide reduced bearing friction losses in combination with smooth and substantially vibration-free shaft rotation in a bearing configuration that is compatible with the relatively high speed and thermal transient conditions of a turbocharger operating environment. In this regard, numerous configurations have been proposed for oil-lubricated sleeve-type journal bearings such as rotationally floating bushings mounted generally at opposite ends of the turbocharger shaft at locations generally and respectively adjacent the turbine and compressor housings. Such sleeve-type bearing systems have additionally required a separate thrust bearing typically in the form of a radial collar on the rotating turbocharger shaft to sustain axial loads during operation. However, such collar-style rotating thrust bearings have been associated with substantial friction losses.
In recent years, improved turbocharger bearing systems using improved anti-friction ball bearings have been proposed. In this regard, angular contact ball bearings have been suggested wherein a pair of angular contact ball bearing units is provided for supporting opposite ends of the high speed turbocharger shaft. See, for example, U.S. Pat. Nos. 6,739,845, 6,877,901, and 7,025,579, all of which are incorporated by reference herein. In such designs, the turbine-end and compressor-end ball bearing units are mounted within a common, generally cylindrical bearing carrier, and are respectively designed to carry thrust loads acting in opposite axial directions, namely, an inboard direction (i.e., with the thrust loads acting axially toward each other). Accordingly, the angular contact bearing units provide both radial and axial thrust bearing functions. At least one of the angular contact ball bearing units is further associated with a spring or tolerance ring for applying an axially outboard-directed thrust pre-load force to the associated bearing unit outer race, thereby accommodating at least some axial migration of the outer race relative to the bearing carrier in response to thermal transients and the like. However, the spring or tolerance ring comprises a relative compliant structure that is difficult to manufacture in a sturdy and rugged form compatible with normal turbocharger operating conditions. In addition, bearing systems using such compliant springs or tolerance rings are typically not well-suited for quick and easy precision installation into a turbocharger center housing.
The present invention relates to an improved turbocharger shaft bearing system of the general type including angular contact ball bearing units, but wherein these ball bearing units are mounted within a rotationally floating bearing cartridge adapted for quick and easy precision installation into a turbocharger center housing or the like, and further wherein the angular contact ball bearings are slightly unloaded, or subjected to a slight negative pre-load, to enhance smooth-running operation with minimal bearing wear while accommodating radial and alternating thrust loads, thermal loads, and transient condition loads during normal operation.